The effect of neutron irradiation on martensite ageing

Following rapid quenching, carbon atoms in the iron-alpha lattice assume positions which in many cases are unfavourable from the energy point of view and diffusion therefore takes place of carbon atoms to more favourable positions. The diffusion proceeds until the atoms collide with other carbon atoms and form either stable pairs or local cumulations, or until they are trapped at dislocations or vacancies. Upon neutron irradiation many point defects - vacancies and interstitial atoms - are formed in iron. Vacancies may form more complex defects, such as divacancies, trivacancies, etc. having a spherical shape. The trapping of the carbon atoms by vacancies results in reducing or, in the extreme case, suppressing martensite ageing. During regeneration, point defects gradually disappear by annihilation, etc. Regeneration at higher temperatures, i.e., when the carbon atoms are released from the vacancies, does not affect martensite ageing. At temperatures higher than 200 degC martensite decomposes and the released carbon atoms appear directly in the form of carbides or carbonitrides without affecting ageing. (J.P.)